<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    * {
      margin: 0;
      padding: 0;
    }

    li {
      list-style: none;
    }

    body {
      background: url(img/1.jpg) no-repeat center top;
    }

    .baidu {
      overflow: hidden;
      margin: 100px auto;
      background-color: #fff;
      width: 410px;
      padding-top: 3px;
    }

    .baidu li {
      float: left;
      margin: 0 1px;
      cursor: pointer;
    }

    .baidu img {
      width: 100px;
    }
  </style>
</head>

<body>
  <ul class="baidu">
    <li><img src="img/1.jpg" alt=""></li>
    <li><img src="img/2.jpg" alt=""></li>
    <li><img src="img/3.jpg" alt=""></li>
    <li><img src="img/4.jpg" alt=""></li>
  </ul>
  <script>
    var imgs = document.querySelector('.baidu').querySelectorAll('img');
    for (var i = 0; i < imgs.length; i++) {
      imgs[i].onclick = function () {
        // this.src 就是我们点击图片的路径
        // 把这个路径 this.src 给body就可以了
        document.body.style.backgroundImage = 'url(' + this.src + ')';
      }
    }
  </script>
</body>

</html>